An elongated flexible insertion instrument of the so-called colonoscope is introduced into the large intestine via the anus and rectum. In order to broaden the examination range, it is desirable for a colonoscope to be able to advance a fore distal end of the insertion instrument as far as the cecum from a depth of the colon. The large intestine includes the sigmoid colon, descending colon, transverse colon and ascending colon, of which the sigmoid colon is in the shape of a three-dimensional loop, taking a very tortuous form in a transitional part to the descending colon. For these reasons, difficulties are usually experienced in introducing a colonoscope into a depth of the colon, passing an insertion instrument of the colonoscope around turns in a three-dimensionally complicated intestinal tract. For passage through such a tortuous path of insertion, the elongated insertion instrument of the colonoscope needs to have a structure which is pliable in a certain degree but stiff enough for transmitting a thrusting force securely from a proximal to fore distal end of the insertion instrument.
In this connection, disclosed in Patent Literature 1 below is an endoscopic insertion instrument which is variable in stiffness in bending directions depending upon the mode of manipulative operation. In an order from a proximal end which is connected to a manipulating head grip, generally an endoscopic insertion instrument is composed of an elongated flexible tube section, an articular flexing section and a rigid distal end section. The flexible tube section is formed of a flexible tubular structure which is flexible in bending directions. In Patent Literature 1, a closely wound stiffness-control coils are extended internally of a flexible tubular structure of a flexible tube section from a proximal end to a fore distal end thereof. Operating wires are passed through the closely wound stiffness-control coils, and a fore end of each operating wire is fixed to a connection ring which is interposed between the flexible tube section and articular flexing section, and a fore end of the stiffness-control coil is fixed to the operating wire. A proximal end of the operating wires is led out of the stiffness-control coil and connected to a push-pull manipulation rod. When the manipulating rod is pulled, adjacent helices of the stiffness-control coil are closed more tightly to each other to stiffen part of the flexible tube section of the insertion instrument. Besides, under the influence of tensioning of the operating wires, the flexible tube section as a whole is compressed and stiffened furthermore.
[Patent Literature 1] Japanese Laid-Open Patent Application 2002-355217